President Donald Trump is considering at least four people to serve as his next chief of staff, after plans for an orderly succession for departing John Kelly fell through.The high-profile hiring search comes at a pivotal time as the Republican president looks to prepare his White House for the twin challenges of securing his re-election and fending off expected congressional investigations once Democrats gain control of the House in 2019.Trump's top pick for the job, Nick Ayers, announced on Sunday that he would instead be leaving the White House, surprising even senior staffers who believed the move was a done deal. Trump is now soliciting input on at least four people, including Office of Management and Budget director Mick Mulvaney and Rep. Mark Meadows, R-N.C., the chair of the conservative House Freedom Caucus.Ayers, who is chief of staff to Vice President Mike Pence, was seen as the favorite for the job when Trump announced on Saturday that Kelly would leave around year's end. But a White House official said on Sunday that Trump and Ayers could not reach agreement on Ayers' length of service and that he would instead assist the president from outside the administration. The officials spoke on condition of anonymity to discuss the sensitive personnel matters.Ayers confirmed the decision in a tweet on Sunday, thanking Trump and Pence for giving him the opportunity to work in the White House."I will be departing at the end of the year but will work with the #MAGA team to advance the cause," he said.Trump offered his own take on the development: "I am in the process of interviewing some really great people for the position of White House Chief of Staff. Fake News has been saying with certainty it was Nick Ayers, a spectacular person who will always be with our #MAGA agenda. I will be making a decision soon!"Even senior White House officials were caught off guard Sunday by the news of Ayers' departure. No obvious successor to Kelly was in sight, and there was some fretting that Trump may not be able to fill the job by the time Kelly leaves.Ayers and Trump had discussed the job for months, making the breakdown on Sunday all the more surprising. Trump said on Saturday that he expected to announce a replacement for Kelly in a day or two. But with Ayers no longer waiting in the wings, Trump may now take until the end of the year, according to a person familiar with the president's thinking.And it remains unclear who wants the job.Mulvaney, the budget director, was not interested in becoming chief of staff, according to a person close to him who spoke on the condition of anonymity. Mulvaney has been saying for almost two months now that he would be more interested in becoming commerce or treasury secretary if that would be helpful to the president, the person said.Also among those thought to be in the mix were Treasury Secretary Steve Mnuchin and US Trade Representative Robert Lighthizer, who said in a CBS interview that he hadn't spoken to anyone at the White House about the job and was "entirely focused" on his position.The White House official said that, while the president likes Lighthizer, he is reluctant to move him from his current post because of the ongoing high-stakes trade negotiations with China and others.And a person familiar with Mnuchin's thinking said he, too, was happy with his work at Treasury and had not sought the job of chief of staff.Acting Attorney General Matthew Whitaker and Trump's former deputy campaign manager David Bossie were also among the names being floated by some close to the White House on Sunday.Trump's administration has set records for staff turnover, and he has often struggled to attract experienced political professionals, a challenge that has grown more difficult by the upcoming threat of costly Democratic oversight investigations and an uncertain political environment.Democrats, who will be assuming control of the House of Representatives next year, are expected to take full advantage of their new subpoena power to investigate everything from the actions of Trump administration officials to the president's business dealings, flooding the White House with inquiries.In any administration, the role of White House chief of staff is split between the responsibilities of supervising the White House and managing the man sitting in the Oval Office. Striking that balance in the turbulent times of Trump has bedeviled Kelly and his predecessor, Reince Priebus, and will be the defining challenge for whoever is selected next.Kelly, whose last day on the job is set to be January 2, had been credited with imposing order on a chaotic West Wing after his arrival in June 2017 from his post as homeland security secretary. But his iron fist also alienated some longtime Trump allies, and over time he grew increasingly isolated.Trump wants his next chief of staff to hold the job through the 2020 election, the officials said. Ayers, who has young triplets, had long planned to leave the administration at the end of the year and had only agreed to serve in an interim basis through next spring.Ayers had earned the backing of the president's influential daughter and son-in-law, White House advisers Ivanka Trump and Jared Kushner, but was viewed warily by other aides.Ayers will run a pro-Trump super PAC, according to a person familiar with his plans who was not authorized to discuss them by name.Pence's deputy chief of staff, Jarrod Agen, is expected to assume Ayers' role for the vice president.